You might also have a clogged cat or muffler. Crawl underneath, and hit the cat...see if it sounds like the internals are loose on it...it will rattle if it's bad. Not sure about the brakes...I was going to say that it might be a bad vacuum booster, but as I sit here, I can't for the life of me remember if there's a conventional vacuum booster on a Fiero or not, and my car isn't here to check. I had a Shelby Mustang that had that same problem, though, and what it turned out to be was a ruptured diaphragm in the vacuum booster.
Mike...86 GT 4 Speed
"Sucks to be me..."